The present invention relates to microstructured pattern inspection method, particularly, to a method of inspecting the microstructured patterns, such as contact holes and linear patterns, that are formed on semiconductor wafers with the photolithography that uses an optical exposure apparatus such as a stepper.
In the manufacture of semiconductors, photolithography is used to form patterns on semiconductor wafers. The formation of these patterns most commonly uses the reduction projection alignment method that applies an apparatus in which a reticle formed by enlarging the circuit patterns for several chips is used for reduction projection alignment (hereinafter, this apparatus is referred to as the stepper). In the reduction projection alignment method using the stepper, a reduced image of the mask pattern of the reticle is exposed to light so as to be projected and formed on the photoresist coating of the wafer, with the result that a resist pattern, a copy of the reticle mask pattern, is formed on that wafer by processing chemically the photosensitized photoresist coating. The patterns for several chips that have been formed on the reticle can be copied with a single shot (exposure). This procedure is xe2x80x9cstepped and repeatedxe2x80x9d to copy more such patterns on the wafer.
An example of forming contact holes on the insulating film of the wafer is described below. First, a photoresist coating is formed on the insulating film. Next, the photoresist coating undergoes exposure using a reticle provided with a pattern of contact holes of the design size and arrangement, and then undergoes chemical processing. After this, contact hole patterns passing through the insulating film can be formed on the wafer by performing processes such as etching, and in this etching process, the photoresist coating that has been created by copying the required pattern functions as a mask.
To ensure that the stepper forms patterns on the wafer as described above, the microstructured patterns on the dimensionally enlarged reticle must undergo reduction projection alignment on the wafer through projection optics. The surface and bottom of the exposed layer (photoresist coating) of the patterns that have been exposed to light in the reduction projection alignment process occasionally differ in size, shape, position, and other factors.
The first main cause of these differences is a combination of defects in the wafer material and defects in the workmanship of the substance exposed to light on the wafer, such as a resist. The warping, distortion, deflection, and the like, of the wafer itself can occur during its manufacture or according to the subsequent elapse of time or the particular ambient environmental conditions such as temperature, and these defects affect optical interference. The shapes of the patterns formed will also be affected by the nature of the substance to be used as a resist, and by the resist coating thickness, coating status, and other factors. Such deviations (from design specifications) in terms of the forming positions and dimensions at the exposed surface and bottom of the microstructured patterns due to the characteristics of the exposed substance (hereinafter, these deviations are collectively called xe2x80x9cdislocationsxe2x80x9d) are usually distributed over a wide range in a specific area of the wafer, and with a fixed tendency.
The second main cause is such insufficiency in the performance of the optics used in the stepper as schematized in FIG. 2. As shown in FIG. 2(a), no problems occur in the vicinity of the reticle center. As shown in FIG. 2(b), however, if light is emitted obliquely to the surface of the wafer, lens aberration, such as astigmatism or comatic aberration, will occur at the edges of the reticle. Dislocation due to such aberration mainly appears within a single-shot area, radially from its central position and with a fixed tendency.
The third main cause is a defect in the nature of the optics of the stepper, that is to say, a shift in focal position (defocusing), which arises from the fact that the lenses in the optics used for exposure suffer deformation due to the heat generated during exposure (this event is called xe2x80x9clens heatingxe2x80x9d).
The fourth main cause is a defect in the performance of the optics of the stepper. If the optics of the stepper has any inclined parts such as lens, since the emitted light enters laterally, the exposure pattern within a single-shot area skews in a fixed direction.
Differences between the design specifications and actually formed patterns are mainly caused by the four factors described above. The first problem resulting from these differences is that dislocation occurs between the patterns that were formed on the surface and bottom of the photoresist. Similarly, there also occurs misalignment with respect to the pattern in the lower layer or upper layer of the insulator, due to the axial and position offsets between the design specifications and actually exposed patterns. Axial or position offsets in contact hole patterns reduce the area of the hole, thus increasing electrical resistance, and finally leading to deteriorated semiconductor performance. In some cases, the semiconductor loses electroconductivity, which is a critical defect in the semiconductor device itself.
With respect to these problems, at present, exposure accuracy at the bottom area of the microstructured patterns is usually evaluated by calculating the area of the bottom. However, there is no established method for evaluating quantitatively the optics of the stepper, the wafer, or the like, from the quantity or direction of pattern dislocation or from these factors.
The present invention is therefore intended to provide a method of evaluating each microstructured pattern of a semiconductor by calculating as a dislocation vector the relationship in position between the surface and bottom of the photoresist on the microstructured pattern. The present invention is also intended to provide a method of evaluating exposure accuracy quantitatively on a single-shot, single-chip, or wafer-by-wafer basis, or a method of evaluating each section of the pattern exposure system, detecting abnormality, and issuing a related warning.
During microstructured pattern evaluation based on the present invention, the formation status of the patterns on the surface of the exposed layer (hereinafter, simply called the surface layer) and at the bottom of the exposed layer (hereinafter, simply called the bottom layer) and the relationship in position between the surface layer and the bottom layer are analyzed, then the relative dislocation between both layers is calculated as a dislocation vector, and this vector is displayed on the screen of the corresponding apparatus. Also, a warning will be issued if the dislocation vector oversteps the dislocation tolerance that has been set beforehand. In addition, the exposure system, the wafer, and other targets can be evaluated by classifying calculated characteristic quantities according to the particular tendency and characterizing each single-shot, single-chip, or wafer area.
That is to say, according to the present invention, the microstructured pattern inspection method for inspecting the microstructured patterns formed on the thin coating of a substrate through pattern optical exposure is characterized in that said inspection method comprises a process for acquiring images of the microstructured patterns formed on said thin coating, a process for identifying both the shape of the microstructured pattern on the surface of said thin coating and the shape of the microstructured pattern at the bottom of said thin coating, from said images, and a process for detecting the dislocation between the two microstructured patterns that have been identified in the third process mentioned above. The shapes of the microstructured patterns can be identified by detecting the profiles of the patterns.
For circular microstructured patterns such as contact hole patterns, misalignment between the gravity center of the circular pattern on the surface of a thin coating and the gravity center of the circular pattern at the bottom of the thin coating is detected as a dislocation. For linear microstructured patterns,
misalignment between the central axis of the linear pattern on the surface of said thin coating, and the central axis of the linear pattern at the bottom of said thin coating, is detected as said dislocation.
The dislocation of microstructured patterns can be visually and easily recognized by displaying at the patterns an arrow indicating the size and direction of the dislocation. It is desirable that the profiles of the microstructured patterns be displayed as marks such as approximate curves or discontinuous dots.
A microstructured pattern inspection method based on the present invention can further comprise a process in which said dislocation is detected at a plurality of positions within the required zone, and a process in which a dislocation that characterizes said zone is detected through statistical processing of the dislocation at said multiple positions. In this case, the dislocation of the entire microstructured patterns in the corresponding zone can be visually and easily recognized by displaying in that zone the appropriate arrow according to the particular size and direction of the dislocation characterizing the zone. This zone can be either a single-shot area or a single-chip area.
Since a process for comparing the distribution tendency of the dislocation at said multiple positions, and the distribution tendency of the dislocation estimated to occur if trouble is detected in the corresponding microstructured pattern forming apparatus, is also included in the microstructured pattern inspection method described above, trouble with the microstructured pattern forming apparatus can be detected.
In addition, according to the present invention, the microstructured pattern inspection method for inspecting the microstructured patterns formed on the thin coating of a substrate through pattern optical exposure is characterized in that said inspection method comprises a process for acquiring images of the microstructured patterns formed on said thin coating, a process for identifying the shapes of the microstructured patterns from said images, and a process for categorizing the corresponding microstructured patterns by the characteristic quantities of the respective shapes.
This microstructured pattern inspection method can also include a-process in which the corresponding microstructured patterns are categorized at a plurality of positions within a single-shot or single-chip area, and a process in which the categories of the microstructured patterns characterizing said single-shot or single-chip area are determined through statistical processing of the categorizing results obtained at said multiple positions. During statistical processing of the categorizing results, the quantity of inspection within, for example, each shot or each chip, and the number of microstructured patterns belonging to a specific category are compared and the highest pattern in terms of rate is characterized as a typical pattern at the particular position. Overall characteristics can be visually and easily identified by displaying a single-shot or single-chip zone in the appropriate color according to the particular category of the microstructured patterns characterizing the corresponding single-shot or single-chip area.
Under the present invention, not only the edge positions corresponding to the surface and bottom of the exposed layers of the contact hole and/or linear patterns are displayed, but also the dislocation between the patterns on both layers is displayed as a dislocation vector at the same time. And a warning will be issued if the dislocation vector oversteps a predetermined tolerance. Thus, it becomes easy to automate the evaluation of microstructured pattern exposure accuracy and to confirm the exposure accuracy. In addition, not only the edge positions corresponding to the surface and bottom of the exposed layers of the contact hole and/or linear patterns are displayed, but also the characteristic quantities of exposed patterns in terms of shape are calculated at the same time. And a warning will be issued if these characteristic quantities overstep their tolerances. Thus, it becomes easy to automate the evaluation of microstructured pattern exposure accuracy and to confirm the exposure accuracy. In addition, it is valid to analyze the dislocation vector and characteristic quantities in combined form. Furthermore, useful data for trouble detection in the optics of the stepper, for statistical evaluation of thermal stresses due to thermal treatment over a wide range, and for statistical evaluation of lens aberration such as astigmatism or comatic aberration, can be collected by analyzing the distributions of the characteristic quantities of dislocation vectors and/or microstructured patterns over a broader area such as a single-chip or single-shot area or the entire wafer. And the inspection of said microstructured patterns to any multiple processes enables collected data to be fed back to subsequent processes.